IMO The kiss could have been a one time thing all you can do about this is trust your instincts about this.
I was in the Air Force for over 6 years--usually I worked 44-50 hours a week (M-F and occassionally on the weekend) plus quarterly exercises where I worked from 6 PM -6 AM for 2 or 3 days. I had a job where I didn't deploy. Some military jobs involve a lot of deployments where you travel to training exercises or real life missions lasting from 3 days to 8 weeks or long term deployments away from your spouse/family for an entire 1-2 years. Though it did not happen that often, I traveled for work and training occassionally. Everyone has some training they travel to during different parts of their career. Since he is going into the military, if you marry, there will be times when your relationship will continue to be long distance so trusting him will be important.
Good luck figuring this out. Hugs.